1. Field of the Invention
The invention relates to an electromagnetic component; and more particularly, to an electromagnetic component appointed for use in a dynamoelectric machine or inductive device, and a method for the manufacture of such a component.
2. Description of the Prior Art
Multi-pole rotating electromechanical or dynamoelectric machines, such as motors, generators, re-gen motors, alternators, brakes and magnetic bearings typically employ one or more electromagnetic components, which may include a stationary component known as a stator and a rotating component known as a rotor. Motors rotate by producing a rotating magnetic field pattern in an electromagnetic component that causes the rotor to follow the rotation of this field pattern. As the frequency varies, the speed of the rotor varies. To increase the shaft speed of the motor, the frequency of the input source must be increased. For certain applications, like electric or hybrid cars, electric motors operable at high speed but also having high efficiency are especially desirable.
Adjacent faces of the rotor and stator in a rotating machine are separated by a small airgap traversed by magnetic flux linking the rotor and stator. It will be understood by those skilled in the art that a rotating machine may comprise plural, mechanically connected rotors and plural stators. Virtually all rotating machines are conventionally classifiable as being either radial or axial airgap types. A radial airgap type is one in which the rotor and stator are separated radially and the traversing magnetic flux is directed predominantly perpendicular to the axis of rotation of the rotor. In an axial airgap device, the rotor and stator are axially separated and the flux traversal is predominantly parallel to the rotational axis.
Except for certain specialized types, motors and generators generally employ soft magnetic materials of one or more types. By “soft magnetic material” is meant one that is easily and efficiently magnetized and demagnetized. The energy that is inevitably dissipated in a magnetic material during each magnetization cycle is termed hysteresis loss or core loss. The magnitude of hysteresis loss is a function both of the excitation amplitude and frequency. A soft magnetic material further exhibits high permeability and low magnetic coercivity. Motors and generators also include a source of magnetomotive force, which can be provided either by one or more permanent magnets or by additional soft magnetic material encircled by current-carrying windings. By “permanent magnet material,” also called “hard magnetic material,” is meant a magnetic material that has a high magnetic coercivity and strongly retains its magnetization and resists being demagnetized. Depending on the type of machine, the permanent and soft magnetic materials may be disposed either on the rotor or stator.
By far, the preponderance of dynamoelectric machines currently produced use as soft magnetic material various grades of electrical or motor steels, which are alloys of Fe with one or more alloying elements, especially including Si, P, C, and Al. Most commonly, Si is a predominant alloying element. While it is generally believed that motors and generators having rotors constructed with advanced permanent magnet material and stators having cores made with advanced, low-loss soft materials, such as amorphous metal, have the potential to provide substantially higher efficiencies and power densities compared to conventional radial airgap motors and generators, there has been little success in building such machines of either axial or radial airgap type. Previous attempts at incorporating amorphous material into conventional radial or axial airgap machines have been largely unsuccessful commercially.
Electromagnetic components are also widely used in static inductive devices, such as transformers and inductors, which are essential components of a wide variety of modern electrical and electronic equipment. Most of these devices employ a core comprising a soft ferromagnetic material and one or more electrical windings that encircle the core. Inductors generally employ a single winding with two terminals, and serve as filters and energy storage devices. Transformers generally have two or more windings. They transform voltages from one level to at least one other desired level, and electrically isolate different portions of an overall electric circuit. Inductive devices are available in widely varying sizes with correspondingly varying power capacities. Different types of inductive devices are optimized for operation at frequencies over a very wide range, from DC to GHz. Virtually every known type of soft magnetic material finds application in the construction of inductive devices. Selection of a particular soft magnetic material depends on the combination of properties needed, the availability of the material in a form that lends itself to efficient manufacture, and the volume and cost required to serve a given market. In general, a desirable soft ferromagnetic core material has high saturation induction Bsat to minimize core size, and low coercivity Hc, high magnetic permeability μ, and low core loss to maximize efficiency.
The construction of electromagnetic components for high frequency electric devices, including both static and rotating devices, is problematic. Components employing iron or steel as a soft magnetic material to concentrate and shape magnetic flux are quite commonly used. However, at high frequencies, conventional iron or steel components are no longer practical. The core losses of the iron or steel components increase with increasing excitation frequency, reducing overall device efficiency. Additionally, at very high exciting frequencies, the component may become extremely hot, cannot be cooled by any reasonably acceptable means, and may cause device failure. Many present dynamoelectric machines must operate at high rotational speed and thus would benefit from use of a high synchronous exciting frequency, by which is meant a frequency greater than 400 Hz.
The requirements for static devices are often even greater, as power conditioning electronic systems now commonly employ switch-mode circuit topologies in which transformers and inductors operable at frequencies of 1 to 200 kHz are essential. In some circuits, operation at up to 1 MHz or more is desired. Accordingly, satisfactory components and methods for the fabrication thereof are highly sought.
Components such as motors and small to moderate size inductors and transformers for electrical and electronic devices often are constructed using laminations stamped or punched from various grades of magnetic steel supplied in sheets having thickness as low as 100 μm. The laminations are generally stacked and secured and subsequently wound with the requisite one or more electrical windings that typically comprise high conductivity copper or aluminum wire. These laminations are commonly employed in cores with a variety of known shapes. Dynamoelectric device components typically are formed with plural radially directed teeth and adjacent slots to accommodate phase windings that encircle the teeth.
Many of the shapes used for inductors and transformers are assembled from constituent components which have the general form of certain block letters, such as “C,” “U,” “E,” and “I,” by which the components are often identified. The assembled shape may further be denoted by the letters reflecting the constituent components; for example, an “E-I” shape would be made by assembling an “E” component with an “I” component. Other widely used assembled shapes include “E-E,” “C-I,” and “C-C.” Constituent components for prior art cores of these shapes have been constructed both of laminated sheets of stamped conventional crystalline ferromagnetic metal and of machined bulk soft ferrite blocks.
Advanced magnetic materials, including known amorphous and nanocrystalline metal alloys and optimized Fe-based crystalline materials. In particular, such materials are easy to magnetize and demagnetize, which means an electromagnetic component made therewith would have low power loss, low temperature rise at high frequency, extremely fast magnetization and easy conversion of electrical to mechanical energy. An electromagnetic component made of such a metal would generate less core losses and be able to operate at much higher frequencies, resulting in devices of exceptional efficiency and power density.
Nevertheless, certain of the physical properties of these advanced materials make conventional fabrication techniques difficult or impossible. Amorphous and nanocrystalline metal alloy is typically supplied as a thin, continuous ribbon having a uniform ribbon width. However, these metals are thinner and harder than virtually all conventional metallic soft magnetic alloys, so conventional stamping or punching of laminations causes excessive wear on fabrication tools and dies, leading to rapid failure. The resulting increase in the tooling and manufacturing costs makes fabricating bulk magnetic components using such conventional techniques commercially impractical. Various manufacturing techniques have been attempted by industry, including wire electrical discharge machining, electrochemical creep grinding, conventional electrical discharge machining, cutting, stamping, acid etching and fine blanking. None has proven satisfactory for reasons including cost-effectiveness, manufacturing repeatability, or process cycle time. The thinness of amorphous metals also translates into an increased number of laminations needed to form a component with a given cross-section and thickness, further increasing the total cost of an amorphous metal magnetic component. Machining techniques used for shaping ferrite blocks are also not generally suited for processing amorphous metals.
The properties of amorphous and nanocrystalline metal alloys are often optimized by an annealing treatment. However, the annealing generally renders the metal very brittle, further complicating conventional manufacturing processes. As a result of the aforementioned difficulties, techniques that are widely and readily used to form shaped laminations of conventional materials, including silicon steel and other similar metallic sheet-form FeNi- and FeCo-based crystalline alloys, have not been found suitable for manufacturing amorphous and nanocrystalline metal devices and components.
This inability to fabricate complex three dimensional shapes from soft magnetic ribbon while retaining satisfactory magnetic properties has been a significant impediment, for both dynamoelectric and static applications that require high efficiencies and low loss components. A production method that is cost effective, end use functional, and high volume capable, while also providing substantial design flexibility for end use requirements, is highly desirable. Amorphous metals thus have not been accepted in the marketplace for many devices; this is so, notwithstanding the great potential for improvements in size, weight, and energy efficiency that in principle would be realized from the use of a high induction, low loss material.
For electronic applications such as saturable reactors and some chokes, amorphous and nanocrystalline metal alloys have been employed in the form of spirally wound, round toroidal cores. Devices in this form are available commercially with diameters typically ranging from a few millimeters to a few centimeters and are commonly used in switch-mode power supplies providing up to several hundred volt-amperes (VA). This core configuration affords a completely closed magnetic circuit, with negligible demagnetizing factor. However, in order to achieve a desired energy storage capability, many inductors include a magnetic circuit with a discrete air gap. The presence of the gap results in a non-negligible demagnetizing factor and an associated shape anisotropy that are manifested in a sheared magnetization (B-H) loop. The shape anisotropy may be much higher than the possible induced magnetic anisotropy, increasing the energy storage capacity proportionately. Toroidal cores with discrete air gaps and conventional material have been proposed for such energy storage applications. However, the gapped toroidal geometry affords only minimal design flexibility. It is generally difficult or impossible for a device user to adjust the gap so as to select a desired degree of shearing and energy storage. In addition, the equipment needed to apply windings to a toroidal core is more complicated, expensive, and difficult to operate than comparable winding equipment for laminated cores. Oftentimes a core of toroidal geometry cannot be used in a high current application, because the heavy gage wire dictated by the rated current cannot be bent to the extent needed in the winding of a toroid. In addition, toroidal designs have only a single magnetic circuit. As a result, they are not well suited and are difficult to adapt for polyphase transformers and inductors, including especially common three-phase devices. Other configurations more amenable to easy manufacture and application are thus sought.
Moreover, the stresses inherent in a strip-wound toroidal core give rise to certain problems. The winding inherently places the outside surface of the strip in tension and the inside in compression. Additional stress is contributed by the linear tension needed to insure smooth winding. As a consequence of magnetostriction, a wound toroid typically exhibits magnetic properties that are inferior to those of the same strip measured in a flat strip configuration. Annealing in general is able to relieve only a portion of the stress, so only a part of the degradation is eliminated. In addition, gapping a wound toroid frequently causes additional problems. Any residual hoop stress in the wound structure is at least partially removed on gapping. In practice the net hoop stress is not predictable and may be either compressive or tensile. Therefore the actual gap tends to close or open in the respective cases by an unpredictable amount as required to establish a new stress equilibrium. Therefore, the final gap is generally different from the intended gap, absent corrective measures. Since the magnetic reluctance of the core is determined largely by the gap, the magnetic properties of finished cores are often difficult to reproduce on a consistent basis in the course of high-volume production.
Amorphous metals have also been used in transformers for much higher power devices, such as distribution transformers for the electric power grid that have nameplate ratings of 10 kVA to 1 MVA or more. The cores for these transformers are often formed in a step-lap wound, generally rectangular configuration. In one common construction method, the rectangular core is first formed and annealed. The core is then unlaced to allow pre-formed windings to be slipped over the long legs of the core. Following incorporation of the pre-formed windings, the layers are relaced and secured. A typical process for constructing a distribution transformer in this manner is set forth in U.S. Pat. No. 4,734,975 to Ballard et al. Such a process understandably entails significant manual labor and manipulation steps involving brittle annealed amorphous metal ribbons. These steps are especially tedious and difficult to accomplish with cores smaller than 10 kVA. Furthermore, in this configuration, the cores are not readily susceptible to controllable introduction of an air gap, which is needed for many inductor applications. It is also difficult with step-lap core configurations to provide fully balanced polyphase devices, i.e., devices in which the electrical characteristics obtained at the terminals for each of the phases is substantially identical.
Accordingly, there remains a need in the art for component fabrication methods that permit manufacture of electric machines that are highly compact, efficient and reliable. Especially desired are methods that take full advantage of the specific characteristics associated with low-loss material, thus eliminating many of the disadvantages associated with conventional components. Ideally, the benefits include one or more of: efficient use of the soft magnetic material; improved electrical efficiency; and reliable, economical, and rapid large-scale production.